Steerable drilling system

ABSTRACT

A steerable drilling system comprises a bias unit  14  operable by means of a control valve arrangement  22 , the control valve arrangement  22  comprising a rotary valve  24  having a control shaft  28  the angular position of which is controlled by a motor  30.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to UK Application No. 0509153 3 filed on May 5th 2005.

BACKGROUND OF INVENTION

1. FIELD OF THE INVENTION

This invention related to a steerable drilling system for use in the formation of boreholes, for example for use in hydrocarbon extraction.

2. DESCRIPTION OF THE RELATED ART

One known form of steerable drilling system includes a bias unit having several bias pads mounted thereon. Each bias pad is moveable from a retracted position to an extended position by means of an associated piston to which drilling fluid under pressure is supplied through a control valve arrangement. The bias pads are arranged such that, when in their extended positions, they engage the adjacent part of the wall of the borehole and apply a sideways acting load to the bias unit. It will be appreciated that if the drill bit is connected to the bias unit, the application of such a load to the bias unit will apply a similar load to the drill bit thereby urging the bit to form a desired deviation in the borehole.

The control valve arrangement typically comprises a rotary valve arranged to supply drilling fluid to the pistons in turn. A control shaft of the rotary valve is connected to a suitably controlled roll stabilised platform so that the control shaft and associated part of the valve are held in a substantially geostationary manner.

The provision of a roll stabilised platform results in the control valve arrangement being of fairly complex form and also being of fairly large dimensions.

SUMMARY OF INVENTION

According to the present invention there is provided a steerable drilling system comprising a bias unit operable by means of a control valve arrangement, the control valve arrangement comprising a rotary valve having a control shaft the angular position of which is controlled by a motor.

The motor is conveniently an electrically operated motor. Preferably the operation of the motor is controlled by a control unit, conveniently using information relating to the orientation of the bias unit and the angular position of the control shaft relative to the bias unit.

The information relating to the orientation of the bias unit may be derived from the outputs of accelerometers and/or magnetometers mounted upon, carried by or associated with the bias unit. The information relating to the angular position of the control shaft relative to the bias unit may be derived from the output of an angle transducer arranged to monitor the control shaft.

The invention also relates to a control valve arrangement suitable for use in such a drilling system, the control valve arrangement comprising a rotary valve having a control shaft the angular position of which is controlled by a motor.

BRIEF DESCRIPTION OF DRAWINGS

The invention will further be described, by way of example, with reference to the accompanying drawings to which:

FIG. 1 is a diagrammatic view of a drilling system in accordance with one embodiment of the invention, and

FIG. 2 is a diagram illustrating part of the drilling system of FIG. 1.

DETAILED DESCRIPTION

The drilling system illustrated diagrammatically in FIG. 1 comprises a drill bit 10 connected to a drilling collar 12 forming the housing of a bias unit 14. A plurality of bias pads 16 are pivotally connected to the collar 12, each bias pad 16 including a piston region 18 slidable within a respective bore 20 provided in the collar 12 so as to be moveable between a retracted position and an extended position. In the retracted position, the bias pad 16 lies adjacent the periphery of the collar 12. When in its extended position, the pad 16 projects from the circumference of the collar 12 and, in use, will engage an adjacent part of the wall of the borehole being formed so as to apply a sideways acting load to the collar 12, and hence to the bit 10.

A control valve arrangement 22 is provided to control the supply of drilling fluid or mud under pressure to the bores 20 within which the piston regions 18 are moveable. The control valve arrangement 22 comprises a rotary valve 24 having an inlet to which fluid under pressure is supplied and a series of outlets, each outlet being connected through appropriate lines 26 to the bores 20. A valve member is rotatably moveable, the valve member being shaped to define a flow path which communicates with the inlet and which communicates with the outlets, in turn, as the valve member is rotated so as to supply fluid under pressure to the bores 20 in turn as the valve member rotates. Depending upon the valve design, there may be times at which fluid is supplied to more than one of the lines. It will be appreciated, therefore, that the bias pads 16 are each moved from their respective retracted positions to their respective extended positions, in turn, as the valve member rotates. The valve member is connected to a control shaft 28, and it will be appreciated that the angular position of the control shaft 28 determine which one or ones of the lines 26, and hence the bores 20 are supplied with fluid under pressure at any given time.

The bias unit 14 and drill bit 10 are connected to an output of a downhole motor so as to be driven for rotation. A load is applied to the bias unit 14 and bit 10 and it will be appreciated that the combination of the application of the load and the rotary motion of the bit causes the bit to gouge, scrape or abrade material from the formation in which the borehole is being formed. Drilling fluid is used to carry away material removed in this manner.

As the bias unit 14 is arranged to rotate, in use, in order to apply a sideways acting load to the bias unit 14 and bit 10 acting in a desired direction so as to cause a desired deviation in the direction of the borehole being formed it will be appreciated that the bias pads 16 need to be removed from their retracted positions to their extended position in a manner synchronised with the rotation of the bias unit 14, and this is achieved by holding the control shaft 28 against rotation relative to the formation.

In accordance with the invention, the control shaft 20 is connected to or forms an extension of the output shaft of a motor 30 the operation of which is controlled by a motor controller 32. As illustrated in FIG. 2, the motor controller is driven from a DC power source generated downhole by means of a turbine 34 and alternator 36. Although it is convenient to use a downhole generated electrical supply to control the operation of the motor controller 32 and motor 30, it will be appreciated that other forms of power source could be used if desired. For example, suitable batteries could be used.

The motor controller 32 is arranged to receive, in real time, tool face measurement signals representative of the orientation of the collar 12. These signals are generated by sensors in the form of an accelerometer 38 and a magnetometer 40, in the illustrated arrangement. However, it will be appreciated that rather than use signals from a plurality of different types of sensor, a single type of sensor could be used if desired. In addition to the signals representative of the orientation of the drilling collar 12, the motor controller 32 is further supplied with signals representative of the angular position of the control shaft 28 relative to the drilling collar 12. These signals are derived from the output of an angle transducer 42.

The information representative of the absolute orientation of the drilling collar 12 is used, in combination with the information representative of the angle of the shaft 28 relative to the drilling collar 12 to determine, substantially in real time, the absolute orientation of the control shaft 28. Using a downhole computer 44, the calculated angular position of the shaft 28 is compared with a desired angular position thereof, and a signal is generated to cause the motor 30 to be driven to cause rotation of the drive shaft 28 to the desired position at any given time. It will be appreciated, therefore, that the bias pads 16 can be driven to their extended positions, in turn, at the desired time to achieve drilling in the desired direction.

It will be appreciated that a range of modifications and alterations to the arrangement described herein may be made within the scope of the invention. It will further be appreciated that the provision of a roll stabilised platform, and the associated disadvantages, are avoided. 

1. A steerable drilling system comprising a bias unit operable by means of a control valve arrangement, the control valve arrangement comprising a rotary valve having a control shaft the angular position of which is controlled by a motor.
 2. A system according to claim 1, wherein the motor is an electrically operated motor.
 3. A system according to claim 2, wherein the operation of the motor is controlled by a control unit.
 4. A system according to claim 3, wherein the control unit controls the motor using information relating to the orientation of the bias unit and the angular position of the control shaft relative to the bias unit.
 5. A system according to claim 4, wherein the control unit further uses information relating to the rotational speed of the bias unit.
 6. A system according to claim 4, wherein the information relating to the orientation of the bias unit is derived from the outputs of accelerometers and/or magnetometers mounted upon, carried by or associated with the bias unit.
 7. A system according to claim 5, wherein rotational speed information is derived from a rotational rate sensor.
 8. A system according to claim 7, wherein the rotational rate sensor comprises a gyroscope.
 9. A system according to claim 4, wherein the information relating to the angular position of the control shaft relative to the bias unit is derived from the output of an angle transducer arranged to monitor the control shaft.
 10. A control valve arrangement for use in a drilling system as claimed in claim 1, the control valve arrangement comprising a rotary valve having a control shaft the angular position of which is controlled by a motor. 